1. Field of the Invention
The present invention relates to a driver for a liquid crystal display device and, more particularly, to a driver for selectively applying voltages to pixels of an active matrix type liquid crystal display device.
2. Description of the Prior Art
As a driving system for a matrix liquid crystal display element, an active matrix driving system is widely known.
For a conventional multi-gradation display liquid crystal display device using, e.g., four reference voltages, a liquid crystal driver output circuit, as in FIG. 1, is used. Four reference voltages V.sub.R3, V.sub.R2, V.sub.R1, and V.sub.R0 are respectively applied to switches SW12, SW13, SW14, and SW15 which are selectively turned on/off and constitute a first stage. Outputs from the adjacent pairs of switches are commonly applied to switches SW16 and SW17 which are alternatively turned on/off and constitute a second stage. Outputs from the switches SW16 and SW17 are commonly applied to an operational amplifier OP2 as a buffer. As a result, the alternatively selected reference voltage V.sub.R0, V.sub.R1, V.sub.R2, or V.sub.R3 is applied from an output terminal d to a data line of an active matrix type liquid crystal display panel (not shown) and is applied to a pixel electrode in accordance with an ON/OFF operation of a thin-film transistor. The selective ON/OFF operations of the switches SW12 to SW17 are controlled by control signals A and B supplied in the form of digital signals. The control signal A has two values, e.g., 5 V and 0 V. The control signal A is level-shifted to an amplitude of 20 V by a level shifter LS1. At the same time, high- or low-level outputs are obtained from output terminals Q and Q of the level shifter LS1 in accordance with the digital value of the control signal A. Two of the four switches SW12 to SW15 are turned on in accordance with the states of the output terminals Q and Q of the level shifter LS1. The control signal B is also a binary signal having values of 5 V and 0 V. The control signal B is level-shifted by a level shifter LS2, and outputs corresponding to its digital value are obtained from output terminals Q and Q. One of the switches SW16 and SW17 is turned on in accordance with the states of the output terminals Q and Q. Upon control based on the two control signals A and B, one of the four reference voltages V.sub.R0, V.sub.R1, R.sub.R2, and V.sub.R3 is applied to the operational amplifier OP2 as a buffer.
In such a conventional liquid crystal driver output circuit, since the buffer constituted by the operational amplifier OP2 connected in the form of a voltage follower is used, the power required for liquid crystal driving is supplied from the buffer, and each of the switches SW12 to SW17 can be satisfactorily operated by a small field-effect transistor. For this reason, low-voltage signals can be used to control the switches SW12 to SW17, and no problem is posed in terms of crosstalk among signal lines. This circuit, however, has the following problems.
Since a current constantly flows in the operational amplifier OP2, the power consumption of the circuit is large. In addition, since an output voltage from the operational amplifier OP2 includes an inherent offset voltage, the driver output (voltage) tends to deviate from a selected drive reference voltage. If such a voltage deviation occurs, the display brightness of the liquid crystal panel changes, and a sharp image cannot be obtained.